What is the Difference Between Langmuir and Bet Isotherm?
🆚 Go to Comparative Table 🆚The main difference between Langmuir and BET (Brunauer–Emmett–Teller) isotherms lies in the adsorption models they describe. Both isotherms are used to describe the adsorption behavior of gases on solid surfaces, but they differ in the way they represent the adsorption process:
- Langmuir Isotherm: This model assumes that only a monolayer of adsorbate can be formed on the surface, with no interaction between adsorbed molecules. It is used to predict linear adsorption at low adsorption levels and depends on the adsorption capacity of the adsorbent. Langmuir isotherm is derived from the Langmuir adsorption theory, which was developed by Irving Langmuir in 1916.
- BET Isotherm: This model considers multilayer formation and extends the Langmuir model to include multilayer adsorption. It is used to measure the specific surface area of the adsorbent and is typically measured using non-corrosive gases like $\ce{N2}$, $\ce{CO2}$, and $\ce{Ar}$. The BET isotherm depends on the size and number of gas molecules adsorbed.
In summary, the Langmuir isotherm describes monolayer molecular adsorption, while the BET isotherm describes multilayer molecular adsorption.
Comparative Table: Langmuir vs Bet Isotherm
The main difference between Langmuir and BET isotherms is that Langmuir isotherm describes monolayer molecular adsorption, while BET isotherm describes multilayer molecular adsorption. Here is a table summarizing the differences between the two isotherms:
| Feature | Langmuir Isotherm | BET Isotherm |
|---|---|---|
| Description | Assumes monolayer adsorption with no interaction between adsorbed molecules. | Describes multilayer adsorption with interactions between adsorbed molecules in each layer. |
| Mathematical Representation | θ = (K * P) / (1 + K * P). | P / (V * (P0 – P)) = (1 / (C * Vm)). |
| Assumptions | - All surface sites have the same adsorption energy for the adsorbate. - Adsorption of the solvent at one site does not influence adsorption at other sites. |
- Gas molecules physically adsorb infinitely on a solid in layers. - Gas molecules react only with the adjacent layers. - Langmuir theory can be applied for each layer. - Enthalpy for adsorption by the first layer is constant and greater than that of the second layer. - Enthalpy of adsorption of the second layer is equal to the enthalpy of liquefaction. |
| Applications | Suitable for analyzing gases adsorbed on surfaces with limited capacity, like solid catalysts. | Used for surfaces with the potential for multilayer formation, such as gas adsorption on porous materials like activated carbon or zeolites. |
Both Langmuir and BET isotherms describe the adsorption behavior of gases on solid surfaces and assume that the adsorption process reaches equilibrium, where the rate of adsorption is equal to the rate of desorption.
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